The attractive forces
due to energy barrier that hold atoms together in a compound is known as
chemical bond. There are various kinds chemical bonds like ionic covalent but
all involve the stableconfiguration of
atom or ion.

Ionic bond:

This
type of chemical bond is proposed by kossel in 1916.

“A chemical bond
which is formed as a result of complete transfer of electron from one atom to
another so that both atom acquire inert gas configuration is called ionic or
electro covalent bond”

The atom, which can
loose electrons, is called electropositive and atom which can gain electron is
called electronegative. These appositively charged ion are held together
electrostatic force of attraction.

Condition For
Ionic Bonding

1- The
electro negativity difference between the combing elements should be greater
than 1.7.

2- The
energy evolved in the third step is greater than the energy absorb in the first
step.

Ionic or
electrovalent bond is generally formed between elements of group 1A, 11A
(metal) and group V1A ,V11A(non metal).this is due to low ionization potential
of metal and high E.A of non metals for e.g.NaCl,CaBr2,MgO,KI.

Explanation:

For
the formation of ionic bond between metal and nonmetal, let us consider the
energy change involve in the formation of sodium chloride. The hole process may
occur in three steps.

1-Ionisation of Atom.

Gaseous sodium atoms
lose an electron and formed cations by absorbing energy equal to ionization
potential.

Na(g)
+ I.P Na+(g)
+ e-

2-Absorption of Electron.

Chlorine atom gains
an electron and converted into anions in this process equal to electron
affinity is released.

Cl(g) +
e- Cl-(g) +
E.A

3-Formation of
Lattice.

The gaseous cations
and anions due to the electrostatic attraction combine together to give stable
ionic crystal.

Na+(g) +
Cl-(g) NaCl(s)

Because the
oppositively charged ions are brought to their position in the crystal lattice
from infinite distance, enough energy is released to make the over all process
energetically favorable, this is known as lattice energy.

Energy of new system
=495-348-788=-641 kj/mole.

The higher the value
lattice energy of the resulting ionic compound the greater the ease of its
formation.

Lattice energy:

“The
energy released when gaseous cation and anion are brought together from
infinite distance to form 1gm mole of solid crystal is called lattice. It is
denoted by “U” and measured kj/mole.”

PROPERTIES OF IONIC COMPOUNDS :

1. Hard solid:
Ionic compounds consist of large no. of oppositively charge ions, which are arrange
in definite pattern at the strong electrostatic forces between ions act in all
direction through the crystal that is why ionic compounds are hard.

2. High melting point due to strong ionic
forces in the crystal, high heat is required to break these forces hence they
possess high melting point.

3. Solubility: Most of the ionic compound
is soluble in water and some other polar solvents because of the strong
electrostatics attractions between the ions and the polar molecules of solvent
causing a release of energy known as “solvation energy” which overcome the high
lattice of ionic compounds in the solution that’s why those are soluble in
water.

4. Electrolytic nature: These are
invariable electrolytes and conduct electricity in molten as well as aqueous
state; this is due to the movement of free ions under the influence of electric
current.

5. An unusual behavior: Some ionic
compounds like sulphate, phosphate, and fluoride of Ca, Sr and Ba are not
soluble even in polar solvent, this is due to their very high value of lattice
energy which is not over come by solvation energy and ions thus do not separate
in free state.

COVALENT BOND:

“It is a chemical
bond which is formed by mutual sharing of electrons between two atoms.”

The concept of covalent
bond was introduced by G.N Lewis in 1916 and was later explained by pauling, in
term of wave function.

Covalent bond is
generally represented by short line (-) between the bonded atoms or (:)
electron pair. Covalent bond can be classified into single double or triple
bond on the basis of number of bonded electron pairs. If there is only one
electron pair between the two atoms, this is called single bond(-) similarly if
2 or 3 bonded electron pair are present by (=) or (=) these are called double
or triple bond respectively.

Example:

Cl – Cl, H-H H-Cl

O=O, N =N

PROPERTIES OF
COVALENT COMPOUNDS:

1- They
exist in separate covalent molecule because particles are electrically neutral
and have less attractive forces.

2- These are volatile in nature
and most of them are gases or liquid

3- They posses low boiling point and
melting point.

4- They are usually insoluble in water and
soluble in organic solvent.

IONIC CHARACTER
OF COVALENT BOND::

When a covalent bond
exists between two non identical atom the bonded electron pair will be
attracted more towards high electronegative atom thus one of its end is
relatively negative and other end is relatively positive; in this way a
negative and positive pole appears on the molecules this is called ionic
character of covalent bond or polarization of covalent bond.

In HCl, the bonded
electron pair id distorted due to the high E.N of Cl atom and probability of
finding the electrons near Cl is greater, hence chloride is slightly negative
and hydrogen is slightly positive.

The covalent bond in
HCl is called partially ionic bond or polarized ionic bond or polarized
covalent bond.

The ionic character
of a covalent molecule depends upon the difference in E.N. of the bonded atom.

HF is more polarized
and ionic due to big difference in EN of H and F atoms.

POLAR COVALENT
BOND:

“It is a covalent
bond which exists, between non identical atoms and is based on the distortion
in shared pair of electron between bonded atoms”.

In a covalent bond
between two unlike atoms, the shared electrons pair will be attracted more
towards high electronegative atom. This permanent displacement of electron
paired towards one atom in a covalent bond. It will develop a fraction of negative
charged (-δ) on one atom and a fraction of positive charge (+δ) on another
atom.

The molecule AB has
some ionic character and is called polar molecule and such type of bond is
called as polar polarized covalent bond.

The extent of ionic
character of polar covalent bond depends on the difference of bonded atoms for
e.g. HF is more polar than HCl because the electron cloud of HF molecule is
shifted towards F atom to a greater extent.

A polar covalent bond
is more stable and stronger due to the polarization of ionic character on the
molecule, which makes the bond distance, and short pulls the atoms closer. Thus
high bond energy is needed to break the bond.

“A
covalent bond which exists between two identical atoms and the electron density
of the shared atoms is equally distributed to around the two nuclei is called
non polar or non polarized or true covalent bond”

When a covalent bond
exists between two similar atoms A and A, the electron will equally be shared
between the two atoms hence bond will have no ionic character.

e.g. H2 , Cl2 , O2 &
N2

A non-polar bond is
relatively week and unstable due to the
long bond distance and less value of bond energy. Non-polar compound are
generally insoluble in water and have low B.P.

The extent of ionic
character of a polar covalent bond depends on the difference of EN of bonded
atoms.

Reason: Why polar bond
is more stronger than non polar?

A polar covalent bond
is much stronger and stable due to the ionic character or polarization of
molecule. The polarity decreases the bond distance and pulls the atoms closer.
Thus high bond energy is required for bond breaking. Non-polar bond on the
other hand is comparatively week because of non-polarization of molecules and
greater bond distance.

HCl is a polar
molecule and its bond dissociation energy is 431 Kj/mole therefore it is more
stronger than Cl2.

Reason: Bond energy
of “HF” is very high.

Partial ionic
character of HF molecule shortens its bond length and due to the high EN value
of fluorine, the electronic cloud is greatly shifted towards fluorine. Thus the
bond distance of HF would be 0.92 ºA less than the expected bond distance (1.01
A) and hence very high energy is required to break it.

BOND ENERGY :

“ The energy required
to break a bond between two atoms in a diatomic molecule is known as bond
energy, it might be taken as energy released in the formation of a bond from
free atoms and measured in Kj/mole”

Bond energy may be
exothermic and endothermic depending upon bond formation or breaking
respectively.

FACTORS
INFLUENCING BOND ENERGY:

1. POLARITY OF MOLECULES:

The presence of ionic character on the
molecule shortens the bond length and atoms are more strongly bonded together,
therefore energy needed to break the bond would be high.

2. BOND LENGTH:

The
distance between two nuclei in a diatomic molecule is called bond length.
Shorter the bond length more firmly is the atoms held and stronger will be the
bond. That is why bond energy will be high.

3. BOND ODER:

Bond energy increases with the
increased order of bond.

DIPOLE MOMENT :

“The quantitative
measurement of concentration of a molecule is known as dipole moment or it is
the tendency of a molecule to orient in an electric field”

The dipole moment
measures the concentration of positive and negative charges in different part
of the molecule and is equal to the product of ionic charge and distance
between the center of positive and negative charges.

Here q = charge on molecule

d = distance between the center of
positive and negative charges.

REPRESENTATION:

Dipole
moment is represented by (→) along with (-) and the direction of arrow is
towards high electronegative atom in the molecule.

MEASUREMENTS:

Dipole
moment is expressed in Debye after the name of introducer but in S.I system the
unit of dipole moment is C.m(coulomb meter).

The relation between debye and C.m is given below

1
Debye = 10-18 esu X cm = 3.335 x 10-30 sm

SIGNIFICANCE:

Dipole
moment is a measurement of degree of polarity of the molecule greater the value
of dipole moment, more polar will be the molecule. does by knowing the value of
dipole moment, one can predict.

i) The strength and stability of bond .

ii) Its melting and boiling point.

Iii) Its solubility in water.

iv) The geometry of molecule i.e it is
linear or angular.

1-DIPOLE MOMENT OF DIATOMIC MOLECULE:

The dipole moment of
homo diatomic molecule is zero because of unavailability of ionic character in
the molecule.

Cl-Cl H-H

(U=0) (U=0)

The hetreodiatomic
molecules posses dipole moment and its value depends upon extent of
polarization of molecule.

2-DIPOLE MOMENT OF POLYATOMIC MOLECULE.

For polyatomic
molecules dipole moment does not only depend up on the polarity of its bond but
it also depends up on the geometry molecule.

Molecule of CO2
is linear and the bond moment of each C= 0 bond is opposite and equal which
cancels out the effect of each other making CO2 molecule and non
polar and its dipole moment is zero.

Similarly molecule of
CS2 is linear and has zero dipole moment.

H2O is
angular molecule the vector some of bond moment of two H---O bonds is equal but
not in opposite direction and it is polar molecule having a specific dipole
moment.

Molecule of CCl4
has symmetrical structure .the four C-Cl bond are polar but vector sum of dipole
moment of all these are cancelled to each other gives a zero dipole moment due
to the symmetrical structure of molecule.

CO-ORDINATE COVALENT BOND:

This type of chemical
bond was suggested by sidwick and defines as “A covalent bond in which the shared
pair of electron is donated by one atom is called coordinate covalent bond”

In this covalent bond both the electrons are
supplied entirely by one atom .it is a less equitable mode of partnership in
which the contribution is one sided.

REASON OF COORDINATES BOND FORMATION :

Element of group VA, VIA and VIIA remain
have lone pair of electrons after utilizing their valences in the covalent
bond. These lone pair electrons can form a new bond with anther atom having
empty orbital, this is as coordinate bond.

REPRESENTATION:

The dative bond is
represented by (→) and its direction is towards electron accepting group.

It may also be
represented by short line (-) but with the inclusion of positive charge (+) for
electron donor and negative charge (-) for electron accepter.

1. They do not ionize in water and are
poor conductors of electricity.

2. They are very sparingly soluble in
water but dissolve in organic solvents.

3 Since
a coordinate linkage is semi-polar, melting and boiling point are higher
than those of purely covalent compounds
but lower than ionic compounds.

Example of coordinate compounds

1. Ammoiumion(NH4+):

In ammonia molecule,
the central atom linked to three H+ atoms and yet N has an unshared pair electrons.
the H+ ion furnished by an acid has no electron to contribute and chain accept
a pair of electrons loaned by N atom. Thus NH3donates
its unshared electrons to H+ forming ammonium ion.

2. Hydronium ion ( H3O+):

The oxygen atom in
water molecule is attached to two atoms by two covalent bonds. There are still
two unshaired pairs of electrons with the O atom. O atom donates one of these
pairs of electron to H+ ion and the hydronium ion is thus formed.

3. Nitromethane( CH3NO2):

The Lewis structure of nitromethane
is shown below.here the N atom has five valence electons, three of which are
used in forming a covalent bond with C atom and two covalent bonds will O atom.
The N atom is still left with two unshared electrons which are denoted to
another O atom.

SIGMA BOND:

“A covalent bond which is formed by head to
head overlapping of atomic orbital is called as sigma bond”

It is generally
established between “S—S”,”S—Px”and Px—Px atomic orbital. sigma bond formation is
based on parallel or linear overlapping of atomic orbital, therefore bond
strength should be maximum and it needs high energy to break a sigma bond.
However the strength of b/w S—S, S—Px and Px—Px are not exactly same.

The S—S overlap is
not so effective due to its spherical charge distribution. P-orbital has
directional charge distribution and longer lobes which cause more effective and
deep overlapping causing short bond length. Thus S—S sigma bond is relatively
weaker than s-p and p-p. The relative bond strength is given as.

Pi BOND:

“A
covalent bond which is formed by lateral or side overlapping of half filled P-
atomic orbital is called as Pi bond”

The overlapping of atomic orbital takes place
perpendicular to inter nuclear axis. In the formation of Pi bond only Py and Pz
orbital take part through side way overlapping, the extent of overlapping is
small and bond formed is weaker.

This theory was
proposed by Heitler and London
in 1927 and later explained by paulling.

This theory gives us
a clear explanation about bond length, bond energy and the strength of covalent
molecules. Main postulates of this are given below.

1- A covalent bond is formed by the linear
overlapping of half filled atomic orbital ` in
which electron pair is in opposite spin.

2-
The pairing of electrons in the molecule should satisfy the Paul’s exclusion
principle and the two electrons would have different values of spin quantum
number.

3.
The electron pair may be localized by the two nuclei. The strength of bond,
bond length and bond energy depend upon the extent of overlapping. Deeply
overlapped orbital form relatively stronger bond.

MOLECULAR
ORBITAL THEORY:

This
theory was proposed by Hund and Huckle in 1930, it explains the bond order and
magnetic property of covalent molecule. It consists of following postulates.

1. The
linear combination of atomic orbital gives two type of molecular orbital called
as bonding and anti bonding molecular orbital.

The
hypothetical process of mixing of different atomic orbital to produce the same
number of equivalent orbital

HAVING SAME SHAPE and energy is known as hybridization,
the orbital so formed are called hybrid orbital”

The
concept of hybridization was introduced by pauling. The type of hybridization
depends upon the number of mixing orbital i.e.SP3,SP2,SP,dSP2,d2SP3
etc.

1- SP3 – HYBRIDIZATION:

It is a mixing of one
s-orbital and three p-orbital to produce four sp3 hybrids orbital is known as
sp3 or tetrahedral hybridization.

Each
sp3 hybrid orbital possesses the character of s and p in the ratio
of 1:3 these are directed at the corner of regular tetrahedron with an angle of
1090 to each other.

EXAMPLE : FORMATION OF METHANE
(CH4):

The
electronic configuration of carbon is given below.

C(z=6) =1s2,2s1,2px1,2py1.

The
2s and three 2p orbital of carbon mixed together to get a set of four equivalent
sp3 hybrid orbital which are located at the corner of regular tetrahedron and
each sp3 orbital of carbon over laps with s orbital of hydrogen from four sigma
bonds.

Other molecules which
show sp3 hybridization are CCl4, SiCl4,SnCl4
etc.

2-SP2 –
HYBRIDIZATION:

The
mixing of one s and two p atomic orbital to produce three sp2 hybrid
orbital is referred to as sp2 or trigonal hybridization.

OR

These
are the set of three hybrid orbital, which arise from the appropriate
combination of one s, and two p orbital.

These
sp2 orbital are coplanar and directed towards the corners of equilateral with
angle of 120º. Each sp2 hybrid orbital has character of S and P in
the ratio of 1:2 sp2 hybrids orbital is identical in shape.

Example:

Formation of ethane (C2H4)

Each carbon in ethene undergoes sp2
hybridization which is coplanar overlap with each other. at an angle of 120º.
Two-sp2 orbital of each carbon overlap with two S orbital while the
other sp2 orbital

The unhybrid Pz orbital of the two-carbon atom
overlap above and below the plane to form π bond.

Boron trifluoride:

It is also an example of
sp2 hybridization. Boron utilizes its 2s,2px and 2py
orbital for the formation of hybrid orbital. These three hybrids orbital overlap with three p orbital from
three different fluorine atoms to form three B—F bonds at an angle of 120º from
each other.

The
mixing of one S and one P atomic orbital is called SP or diagonal
hybridization. The hybrids SP-orbital are collinear at an angle of 180º which
provide a maximum separation. These SP orbital have character of S and P in the
ratio of 1:1.

One
S orbital can combine with one P orbital on the same atom to form two new and
completely equivalent orbital called sp hybrid orbital.

Example:

Formation of ethyne (C2H2)

Each carbon in ethyne undergoes SP
hybridization, which are collinear at an angle of 180º. One sp1 orbital of each
carbon overlap with one s orbital of hydrogen while the other sp orbital overlaps
with each other.

The unhybridized Py and Pz
orbital of the two carbon atoms overlap above and below the plane to form two π
bonds.

ELECTRON
PAIR REPULSION THEORY:

This
theory explains about the geometry of simple covalent molecules and the ions of
non transitional elements. It is introduced by Sidgewick and Powell in
1940.it is based on the repulsion of electron pairs in the valence shell of
central atom in a molecule.

Main
postulates of this theory are given below.

i) The
central atom in a covalent molecule may have two types of electron pairs, the
bond pairs, and lone pairs of electron. These are called as active set of
electron pairs.

ii) These
bond and loan pair of electrons exert repulsive forces to each other try to be as far apart as possible,
hence they orient themselves in space in such a manner that forces of repulsion
between them is minimized.

iii) The
force of repulsion between two bond pair and loan pair is not the same. The order of repulsion is as
follows.

In case of molecule with double or triple bond, the π electron
pairs are not considered to be involved in the repulsion and hence called as
“inactive set” of electron.

iv) The
shape of molecule depends upon total number of active set of electron pair
around the central atom.